1. Field of Invention
The present invention relates to an improved belt that may be used in a belt separation apparatus to separate a particle mixture based on tribo-electric charging of the particles.
2. Discussion of Related Art
Belt separator systems (BSS) are used to separate the constituents of particle mixtures based on the charging of the different constituents by surface contact (i.e. the triboelectric effect). FIG. 1 shows a belt separator system 10 such as is disclosed in commonly-owned U.S. Pat. Nos. 4,839,032 and 4,874,507, which are hereby incorporated by reference in their entirety. One embodiment of belt separator system 10 includes parallel spaced electrodes 12 and 14/16 arranged in a longitudinal direction to define a longitudinal centerline 18, and a belt 20 traveling in the longitudinal direction between the spaced electrodes, parallel to the longitudinal centerline. The belt 20 forms a continuous loop which is driven by a pair of end rollers 22, 24. A particle mixture is loaded onto the belt 20 at a feed area 26 between electrodes 14 and 16. Belt 20 includes counter-current traveling belt segments 28 and 30 moving in opposite directions for transporting the constituents of the particle mixture along the lengths of the electrodes 12 and 14/16. The only moving part of the BSS is the belt 20. The belt is therefore a critical component of the BSS. The belt 20 moves at a high speed, for example, about 40 miles an hour, in an extremely abrasive environment. The two belt segments 28, 30 move in opposite directions, parallel to centerline 18, and thus if they come into contact, the relative velocity is about 80 miles an hour.
Related art belts were previously woven of abrasion resistant 45 monofilament materials. These belts were quite expensive and lasted only about 5 hours. The mode of failure was typically longitudinal wear stripes due to longitudinal wrinkling, that would wear longitudinal holes in the belt such that it would fall apart and catch on itself. The strands would also wear where they crossed and flexed in moving through the separator. The Applicant has made attempts to improve such belts with different materials and different weaves in an attempt to find a woven material with a longer life. These attempts were unsuccessful.
An improvement over woven belts for BSS were belts made of extruded materials, which have better wear resistance than woven belts and may last on the order of about 20 hours in a BSS. An example of such extruded belts is described in commonly-owned U.S. Pat. No. 5,819,946 entitled “Separation System Belt Construction,” which is herein incorporated by reference. Referring to FIG. 2, there is illustrated a schematic drawing of a section of an extruded belt 40. Control of the geometry of extruded belts is desirable, but can be difficult to achieve with extruded belts.
One extruded belt that has been used in BSS is described in commonly-owned U.S. Pat. No. 5,904,253, which is also herein incorporated by reference. Referring to FIG. 3, which illustrates an enlarged portion of the BSS shown in FIG. 1, the directions of the counter-travelling belt segments 28, 30 are shown by arrows 34 and 36, respectively. As illustrated in FIG. 2-3, one example of a desired belt 40 geometry (See FIG. 3) has a leading edge 43 of the belt 40 (See FIG. 2) of the cross direction strands 46 is provided with an acute angle 44.
To improve the life of the extruded belt and to gain better control of the geometry of the belt profiles discussed in U.S. Pat. No. 5,904,253, a method of joining abrasion resistant thermoplastic sheets as described in commonly-owned U.S. Pat. No. 6,942,752, herein incorporated by reference, has been used to produce belts from ultra high molecular weight polyethylene (UHMWPE) sheets. One example of a convenient method for forming the holes and leading edge and trailing edge features of a desired geometry in such UHMWPE sheets is to use a multi-axis machine tool. With this device, a sheet is loaded onto a table and a cutter head is moved across the sheet and each opening in the belt may be cut individually. Through the proper choice of cutting tool, the holes can be cut with leading edge and trailing edge features as desired. It is also to be appreciated that the desired leading edge geometry can be obtained through other forming processes and devices such as molding, punching, machining, water jet cutting, laser cutting, and the like.